Cadavers
This investigation examined 41 legs from 25 Japanese cadavers (mean age at death, 80 ± 12 years; range, 47–96 years; 18 sides from men, 23 from women; 20 right sides, 21 left sides) that had been switched to alcohol after placement in 10% formalin. No knees showed any signs of previous major surgery around the knee or any relevant deformities, and no obvious degeneration was apparent in any specimens. This study was approved by the ethics committee at our institution (approval number: 18071).
Measurement conditions
In the dissection procedure, an isolated knee specimen was prepared by cutting the distal one-third of the femur and the central part of the tibia, and the skin, subcutaneous tissue, and tensor fasciae lateral were removed. For the IFP body, the synovium and meniscus were incised along the anterior edges of the medial collateral ligament (MCL) and lateral collateral ligament (LCL) (Fig. 1A, 1B). Then, in a position of knee flexion, the quadriceps femoris and patella were inverted from proximally to distally, and the ligamentum mucosum was incised (Fig. 1C). The IFP was detached from the anterior surface of the tibia, and the patellar tendon was resected with the patellar tendon attached to the patella and part of the rough surface of the tibia. The meniscus and transverse knee ligament attached to the IFP were then carefully removed. The presence or absence of the IFP proximal was confirmed between the insertional tendons of the vastus medialis/vastus lateralis, the patellar retinaculum, and the synovium (Fig. 2). Regarding the removal of the synovium, only synovium that was not adherent to the IFP proximal and IFP body was carefully removed (Fig. 2A). Quadriceps muscle fiber bundles were removed along the shape of the IFP proximal (Fig. 2B). Part of the tibial tuberosity was carefully removed from the posterior part to remove the most distal end of the IFP body. After measuring the IFP proximal and IFP body, the IFP was removed from the patella, patellar tendon, and patellar retinaculum for IFP volume measurement.
Morphological measurement was performed by one examiner, and the IFP proximal, IFP body length, width, and IFP volume were measured (Fig. 3). The specimen was photographed from behind using a digital camera (Finepix F600EXR; Fujifilm, Tokyo, Japan) with the patella joint surface facing forward without tilting. At the time of measurement, perpendicular lines were drawn in the following parts: the upper end of the patellar articular surface; the most end of the IFP medial and lateral proximal, the lower end of the patellar articular surface, the horizon passing through the distal end of the IFP body, and the medial and lateral ends of the IFP body. IFP proximal length was measured as the distance between the horizontal line passing through the upper end of the patellar joint surface and the horizontal line passing through the lower end of the patellar joint surface when the IFP was connected to the suprapatellar fat pad (SFP). When the IPF did not bind to the SFP, this distance was taken as the distance between the horizontal line passing through the most end of the IFP proximal and the horizontal line passing through the lower end of the patellar articular surface (Fig. 3A). IFP proximal width was measured at the distance of the horizontal line connecting the medial and lateral ends of the IFP proximal length and was measured at three points of the IFP proximal length (Fig. 3B). IFP body length was measured as the distance between the horizontal line passing through the lower end of the patellar joint surface and the horizontal line passing through the most distal end of the IFP body. The IFP body width is the distance of the line connecting the perpendicular lines passing through the inner and outer ends of the IFP body. Image analysis software (Image J; NIH, Bethesda, MD, USA) was used for the measurement, and the mean ± standard deviation from three measurements was calculated. Before measuring volume, the IFP was soaked in a preservation solution for 10 min to prevent the IFP from drying out. For the IFP volume, a 500-ml cylinder (SANPLATEC, Osaka, Japan) was used, and water displacement was recorded in 2.5-ml intervals, and the mean ± standard deviation from three measurements was calculated. When the IFP proximal and SFP were connected, the SFP was removed at the upper end of the patellar joint surface. To prevent a decrease in IFP volume due to drying of the IFP, specimens were immersed in the preservation solution at 30-min intervals for 10 min, and the entire experiment was performed within 3 h.
Knees were classified into 3 types by the morphological characteristics of IFP proximal. Type I was an IFP proximal located on the medial and lateral sides of the patella (Fig. 4A, 4a). Type II was an IFP proximal located only on the medial side of the patella (Fig. 4B, 4b). Type III was absence of the IFP on both medial and lateral sides (Fig. 4C, 4c).
The articular surfaces were graded [25] as macroscopically intact or mildly altered (Grade I) (Fig. 5A), moderately altered (if fissuring or fibrillation was observed, Grade II) (Fig. 5B), or severely altered (if eburnation was present, Grade III) (Fig. 5C).
Statistical analysis
Fisher’s exact test was used for comparisons of classifications of the IFP by sex and laterality, and to compare differences in degenerative grade of the articular surface for each type of category. Multiple comparisons were performed using the Ryan nominal level for post hoc testing. Comparisons of IFP proximal medial length, width (distal, intermediate, proximal) between Types I and II were made using unpaired t-tests. Comparisons of IFP body length, width, and IFP volume in each type were made with one-way repeated-measures analysis of variance, and Tukey’s method. Comparisons of IFP proximal medial and lateral length, width (distal, intermediate, proximal) among Type I were made with paired t-tests. Statistical analyses were performed using SPSS version 26.0 software (SPSS Japan, Tokyo, Japan). The level of significance was p < .05.